4-3.1. Introduction

In higher eukaryotes, gene expression is tissue-specific. Only certain cell types show moderate to high expression of a single gene or a group of genes. For example, the genes encoding globin proteins are expressed only in erythrocyte precursor cells, called reticulocytes. Using this information a target gene can be cloned by isolating the mRNA from a specific tissue. The specific DNA sequences are synthesized as copies from mRNAs of a particular cell type, and cloned into bacteriophage vectors. cDNA (complementary DNA) is produced from a fully transcribed mRNA which contains only the expressed genes of an organism. Clones of such DNA copies of mRNAs are called  cDNA clones.

A cDNA library is a combination of cloned cDNA fragments constituting some portion of the transcriptome of an organism which are inserted into a number of host cells. In eukaryotic cells, the mRNA is spliced before translation into protein. The DNA synthesized from the spliced mRNA doesn't have introns or non-coding regions of the gene. As a result, the protein under expression can be sequenced from the DNA which is the main advantage of cDNA cloning over genomic DNA cloning.

4-3.2. Construction of a cDNA Library

The construction of cDNA library involves following steps-

1.  Isolation of mRNA

2.  Synthesis of first and second strand of cDNA

3.  Incorporation of cDNA into a vector

4.  Cloning of cDNAs